1. Field of the Invention
This invention relates to a method for reducing the permeability of the more permeable zones of a subterranean reservoir having nonuniform permeability, and more particularly, to such a method wherein permeability reduction is caused to occur only after an injected composition or compositions has passed radially into the reservoir a substantial distance from the well.
2. Description of the Prior Art
It has long been recognized that when fluids are passed through a subterranean reservoir having heterogeneous permeability, for example during an enhanced recovery process or during production of fluids from a well, a relatively higher volume of fluids tends to pass through the more permeable strata with little or no fluid passing through the less permeable strata.
It is known to attempt to even out the flow of fluids through such reservoirs, i.e., increase the proportion of the fluids flowing through the less permeable portions of the reservoir, by injecting into the reservoir a fluid which predominantly enters the more permeable strata where it subsequently thickens and/or at least partially sets to reduce the permeability of these more permeable sections and in some cases plug them completely. Thus, a higher proportion of any fluid subsequently passed through the reservoir will flow through the portions of the reservoir which originally were less permeable.
Among the permeability-reducing and/or plug-forming compositions which have been used are aqueous solutions or dispersions of polymers containing a cross-linking agent. For example, U.S. Pat. Nos. 4,009,755 and 4,069,869 to Sandiford each deals with forming plugs in wells wherein there is injected various combinations of: (a) an aqueous solution of a water-soluble polymer, e.g., polyacrylamide, (b) a cross-linking material that reacts with the polymer to form a polymer-containing plug, e.g., a compound of a multivalent metal and a reducing agent, a low molecular weight water-soluble aldehyde or a colloidal hydroxide of a multivalent cation, (c) an aqueous solution of an alkali metal silicate and (d) a gelling agent that reacts with the silicate to form a silicate-containing plug, e.g., acid and acid-forming compounds, water-soluble ammonium salts, lower aldehydes and aluminates. In the compositions described in each of these references, a large amount of alkali metal silicate is present, and this material becomes a part of the plug.
U.S. Pat. No. 3,687,200 to Routson describes a method for controlling the flow of aqueous fluids in a porous subterranean formation using an aqueous colloidal dispersion of a water-insoluble inorganic compound, such as aluminum hydroxide, prepared in an aqueous solution of a synthetic organic polymeric polyelectrolyte, e.g., a polyacrylamide, without a cross-linking agent. In one method of preparing the dispersion, a small amount of an alkali metal hydroxide or of an alkaline buffer is dissolved in the polymer solution. Thereafter, a dilute solution of a suitable metallic salt, such as a soluble salt of aluminum, e.g., sodium aluminate, is added to the polymer solution to precipitate aluminum hydroxide in the form of a stable colloidal dispersion of the insoluble aluminum hydroxide. The resulting composition has a high resistance factor, i.e., a high pressure drop when forced through a permeable core. An alternative method of preparing the dispersion involves addition of a precipitant such as sodium sulfide to the composition instead of the alkali metal hydroxide. Also, in certain cases, the composition containing the colloidal dispersion is generated inside a subterranean formation. Specifically, if the connate water contains soluble sulfide ions, a solution of a metallic salt in an aqueous solution of polymer is injected to form the colloidal insoluble metallic sulfide in the interstices of the formation. Otherwise, the dispersion plugging agent is produced before the composition is injected into the formation, thereby promoting plugging at the well sidewall and in the immediate vicinity of the well rather in the formation a substantial distance from the well.
U.S. Pat. No. 3,841,399 to Ryan describes forming in situ a polymer-thickened aqueous waterflooding composition by injecting into a formation a low viscosity, potentially hydrolyzable, synthetic aqueous emulsion polymer, such as a water dispersion of acrylonitrile copolymerized with ethyl acrylate, and an aqueous solution of sodium hydroxide. The polymer is eventually hydrolyzed in the subterranean formation to form a high viscosity composition. The caustic solution can be added in admixture with the polymer composition or separately following the polymer composition. No cross-linking agent is employed.
U.S. Pat. No. 3,825,067 to Vestal describes a method for dissolving a polyacrylamide polymer in water to form a composition useful as a secondary recovery drive fluid. The powdered polymer is mixed with dilution water, the pH adjusted to 7 to 14 by the addition of a base such as sodium hydroxide or trisodium phosphate, and the resulting solution heated to simultaneously hydrolyze and solubilize the polymer. No cross-linking agent is utilized. The composition does not form a plug in the reservoir or otherwise alter reservoir permeability, but rather functions as a drive fluid to displace oil from the reservoir.
While each of the aforementioned treatments has met with some success in particular applications, the need exists for a further improved well treating process to selectively reduce the permeability of the more permeable strata of a reservoir having heterogeneous permeability, especially in such strata a substantial distance from a well penetrating the strata. Moreover, the prior art processes often encounter practical difficulties. For example, in those processes employing a polymer and cross-linking agent, if the two materials are mixed together in an aqueous solution at the surface, they tend to begin to react immediately. By the time the resulting composition is pumped down a well and into a reservoir, the composition often has already begun to increase in viscosity and may be difficult to force into even the higher permeability strata. The same is true to a lesser extent if separate aqueous solutions of polymer and cross-linking agents are simultaneously pumped down a well and mix as they pass through the tubing. If one of the compositions is pumped first followed by the second, there is less chance of reaction occurring until after the compositions have entered the reservoir. However, mixing of separately injected compositions in a reservoir is difficult to control. What is desired, therefore, is a composition capable of reducing permeability and injectable down a well and into a reservoir as a low viscosity, easily-pumped liquid. Then, after the composition has been injected through the reservoir some distance away from the well, it reacts to form a viscous, permeability-reducing composition or plug.
Accordingly, it is a principal object of the invention to provide a method for reducing the permeability to fluids of the more permeable strata of a reservoir having heterogeneous permeability.
It is a further object of this invention to provide such a method wherein the permeability reduction occurs a substantial distance in all directions from a well penetrating such reservoir.
It is a still further object of this invention to provide such a method wherein the permeability reduction is achieved by injecting into the reservoir via a well a relatively low viscosity aqueous base composition which reacts to form a relatively high viscosity composition and/or plug only after passing a substantial distance through the reservoir.
Another object of this invention is to provide such a method wherein the location of a plug formed in a reservoir can be controlled.
Other objects, advantages and features of the invention will be apparent from the following description and claims.